1. Field of the Invention
This invention relates to a filmless digital camera in which the data representing the images are compressed in such manner as to minimize the creation of artifacts from bright spots or abrupt change in gray level while conserving data storage capacity. More particularly, the invention relates to a filmless camera having a storage capacity for a predetermined number of images and in which the compression algorithm is automatically modified to obtain the maximum fidelity in the reconstituted images while making full utilization of the storage capacity.
2. Description of Related Art
Many different algorithms for compressing image data have been devised to minimize required memory capaicty with minimum loss of image quality. One widely accepted algorithm is the Discrete Cosine Transform (DCT). For many images in different applications that compression method provides acceptable results. However, certain images upon reconstitution display artifacts in portions of the image adjacent to abrupt changes in the gray level. Many other compression methods have been devised but generally have a fixed compression procedure that is applied uniformly to all parts of the image.
Most compression techniques achieve a fairly uniform image quality level by allowing the compression ratio to change. Such techniques are inappropriate for a filmless digital camera that has a fixed amount of digital memory for image storage. The photographer would not know how many pictures could be taken before filling the memory of the camera.
One variation of the Discrete Cosine Transform is known as Adaptive DCT. The goal of the adaptation is to make the quality more uniform over the area of the image. However, an adaptive DCT would aggravate, rather than alleviate, the problem of variable compression ratio.
U.S. Pat. No. 5,034,804 to Sasaki et al. describes a digital camera in which image data from an A/D converter are recorded on a memory card serving as a recording medium. No method for controlling the quantity of recorded data for each image while providing maximum picture quality is suggested. Because of the use of replaceable memory cards, there is no need to tailor the size of the image data to record a predetermined number of images in a fixed memory base.
U.S. Pat. No. 5,016,107 to Sasson et al. also discloses the use of a memory card for recording digital image data. Again, there is no common fixed-limit memory for recording a fixed number of images. No selective compression of the images is disclosed.
U.S. Pat. No. 5,027,214 to Fujimori describes an imaging system in which a data compressing section causes variable-length encoding data compression of the digital signal. The compression results in a variable amount of digital data for different images. These data are recorded in a memory having a fixed capacity. Accordingly, the number of images that can be recorded is not known in advance, because the amount of data resulting from the compression varies from image to image. Computations provided by the Fujimori structure enables the photographer to know whether an electronic image to be newly photographed can be recorded or not. This is a device for telling the photographer he has, in effect, run out of film. Fujimori does not adjust the data recorded from the images in order to fit a predetermined number of images into the available memory. In the Fujimori patent the image data are first pre-processed using a differential pulse code that is followed by a compression step using the discrete cosine transform. Every image is processed in this manner. There is no disclosure of performing a compression operation and making a decision dependent upon the results of that compression to control the manner in which each particular image is to be treated in order to provide maximum quality while accommodating a pre-set number of images in the fixed memory. Fujimori does not suggest using the number of non-zero digital coefficients for selecting subsequent compression procedures.
U.S. Pat. No. 5,068,744 to Ito describes a still camera recording procedure in which the image is divided into data blocks and each block is individually recorded. Picture components, such as the AC and DC components, are individually recorded.
None of the earlier patents discloses the computation of a measure bearing a pre-established relation to the creation of artifacts upon the decompression of the image data as here proposed by applicant.